Birefringence read {11 {11 {11 {11 display and memory device

ABSTRACT

Information to be stored is written into a single crystal of bismuth titanate, by either direct electrodes, photoconductors, or electron beams. Storage is by oppositely switched domains in the poled bismuth titanate crystal. The crystal is positioned between crossed polarizers, positioned 45* from extinction, and rotated about the b axis such that an OFF state by thickness extinction occurs for one of the domains; the opposite domain then being in an ON state. Readout in one embodiment is by monochromatic light. In another embodiment a second bismuth titanate crystal is positioned adjacent the storage crystal and oriented such that its b axis is perpendicular to the b axis of the first crystal to provide a storage system that is read out with white light. In this second embodiment the second crystal makes the total retardation through it and through one domain of the storage crystal zero which gives no transmission of light, that is, an OFF state; the other domain providing an ON state.

X1? 3&80795530 [4 1 Apr. 30, 1974 BIREFRINGENCE READ Bi4Ti 0 DISPLAY AND MEMORY DEVICE [75] Inventors: Theodore E. Luke, Englewood;

Stewart E. Cummins, New Carlisle, both of Ohio [73] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, DC.

[22] Filed: Mar. 6, 1972 [2]] Appl. No.: 232,173

Primary Examiner-John K. Corbin Attorney, Agent, or Firm1-1arry A. Herbert, Jr.; Robert K. Duncan [57] ABSTRACT Information to be stored is written into a single crystal of bismuth titanate, by either direct electrodes, photoconductors, or electron beams. Storage is by oppositely switched domains in the poled bismuth titanate crystal. The crystal is positioned between crossed polarizers, positioned 45 from extinction, and rotated about the b axis such that an OFF state by thickness extinction occurs for one of the domains; the opposite domain then 'being in an ON state. Readout in one embodiment is by monochromatic light. 1n another embodiment a second bismuth titanate crystal is positioned adjacent the storage crystal and oriented such that its b axis is perpendicular to the b axis of the first crystal to provide a storage system that is read out with white light. In this second embodiment the second crystal makes the total retardation through it and throughpne domain of the storage crystal zero which 7 gives no transmissionof light, that is, an OFF state;

the other domain providing an ON state.

- 11 Claims, 11 Drawing Figures [52] US. Cl. 350/150, 340/173.2 [51] Int. Cl. G02f 1/26 [58] Field of Search 350/150; 250/225; 340/1732, 173 LS [56] References Cited UNITED STATES PATENTS 3,680,060 7/1972 Keneman et a1 350/150 UX 2,936,380 5/1960 Anderson 350/150 X 3,661,442 5/1972 Kumada et al.. 350/150 3,614,754 10/1971 Cummins 350/150 X 3,374,473 3/1968 Cummins 350/150 X (Z 1,4- Malvacmeo MHTIC 4 IGHT SC U e Z5.

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1 BIREFRINGENCE READ si,'r o pisPe v AND MEMORY DEVICE BACKGROUND OF THE INVENTION The field of this invention is in ferroelectric information storage devices. The prior art is well exemplified by the following patents: US. Pat. Nos. 2,909,972 patentee R. B. DeLano, Jr.; 2,928,075 and 2,936,380 patentee J. R. Anderson; 3,229,261 patentees E. Fatuzzo et al; and 3,374,473 patentee S. E. Cummins.

Additional technical information and a more complete theoretical discussion of this invention and the comparison of it with the prior art may be found in the applicants published technical papers, A New Method of Optically Reading Domains in Bismuth Titanate for Display and Memory Applications published in IEEE Transactions on Electron Devices Vol. ED-l8, Number 9, September 1971, pp 761-768, and Efficient White-Light Reading of Domain Patterns in Bismuth Titanate published in Ferroelectrics Vol. 3, February 1972 Nos. 2, 3, and 4. For a complete description of the ferroelectric and optical properties of Bi Ti O in terms of the monoclinic crystal symmetry reference is made to one of the coinventors publications Electrical and Optical Properties of Ferroelectric Bi Ti O Single Crystals" in the Journal of Applied Physics, Vol. 39, pp. 2268-2274, April 1968, and his previous US. Pat. No. 3,374,473.

SUMMARY OF THE INVENTION An optical read storage device comprising a c-plate of a single crystal of Bi Ti O positioned between crossed polarizers in which information is read into the crystal in a conventional manner and the crystal oriented in a beam of monochromatic light to provide differential-retardation readout provides a system having relatively high contrast ratios (greater than 100:1), efficiencies of the order of to 15 percent, and resolutions of 50 to 60 lines/mm. By adding a second cplate crystal of a single crystal of Bi Ti O to the foregoing apparatus, which is positioned to compensate one domain in the storage crystal to zero retardation, the stored information may be read out with conventional white-light with contrast ratios of approximately :1 and efficiencies of approximately 16 percent. The devices of the invention are insensitive to optical degradation or depoling which has been a serious limitation to the prior art devices after repeated polarization reversals. For example, typical embodiments of the invention have been operated in fast l to 2p. sec.) pulse switching for over 200 hours without showing any optical degradation when read by either the differentialretardation system or the compensation system.

The invention herein disclosed presents a new system of apparatus providing greatly improved results for optically reading images or other information stored as ferroelectric domain patterns in Bi Ti O single crystals. The previously referenced prior art U.S. Pat. No. 3,374,473, uses the differences in the extinction directions in opposite domains for optically detecting these domains; this necessitates operation near extinction which allows only a small percentage of the reading light beam to pass through the system. The present invention makes uses of differences in birefringence with the crystal oriented out of extinction which allows much more efficient use of the reading light beam. This increased efficiency pennits the direct optical reading of stored images or other information with relatively low, power light sources, and provides a system having a greatly improved optical signal-to-noise ratio compared to the prior art devices. In addition to the direct viewing of images the disclosed system provides for the reading of information as absorption holograms, as opposed to a somewhat related system providing for reading pure phase holograms in'Bi Ti O BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of an embodiment of the invention using a monochromatic light source;

FIG. 2 is a representative axial pictorial view showing the relationships of the crossed polarizers with the orientation of the Bi Ti O crystal;

FIG. 3 is a schematic diagram of an embodiment of the invention using a white light source;

FIGS. 4a, 4b, and 4c are representative sketches showing the optical indicatrix of Bi Ti O for opposite 0 domains;

FIG. 5 is atypical plot (for A 632.8 nm) of the birefringence as a function of tilt angle 6 for opposite domains saith the path of the light in the a-c plane;

FIG. 6is arepresentative plot (for A 632.8 nm) of crystal thickness and tilt angles showing the parameters for thicknessextinctiom- FIG. 7 shows an enlarged view of a specific area of the curvess'hownjn FIG. 6.

FIG. 8 isa plot showing the calculated variation of effective birefringence as a function of the internal tilt angle (0 for the opposite domains and the operating angle of a typical crystal pair; and

FIG. 9 is a typical plot of measured light output for on and off domains and the contrast ratio (CR) for two cases of compensation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The scientific details and theory of operation of the invention are thoroughly set forth in the previously referenced publications, therefore primarily only the data and information necessary to construct the apparatus of the invention will be contained herein. Two embodiments will be described in detail. The first embodiment is depicted schematically in FIG. 1. In this embodiment readout is by monochromatic light with the light output dependent upon the difference in retardation in opposite-polarity domains. This embodiment may be termed the differential retardation or monochromatic light embodiment. The second embodiment depicted schematically in FIG. 3 may be read out by white light. The construction of this embodiment is similar to that of the first except for the positioning of a compensator crystal in the light path. This embodiment may be termed the white light or compensated retardation embodiment. Generally the path of light through embodiments of the invention will be shown as a straight line, however it is to be understood that this is not a requirement and that for convenience of design the light path through the system may be reflected or refracted as is common in other optical systems.

Briefly, bismuth titanate (Bi Ti O a major element in the apparatus of the invention, is a ferroelectric crystal having a large spontaneous polarization (approximately 50 uClcm lying nearly in the plane of the (asgrown) thin flat crystals (i.e., the a b plane). Switching of a component of P, (spontaneous polarization) along the c axis (normal to the flat plates) gives a hysteresis loop with approximately 4 uC/cm of spontaneous polarization being switched. This type of switching is unique to Bi Ti O and results in a large difference in the optical properties of domains that differ only in the c axis component of polarization. The optical properties are summarized in FIGS. 4a, 4b, and 4c. The optical indicatrix is a triaxial ellipsoid which shows the refractive index as a function of direction in the crystal. This indicatrix determines the optical properties such as the extinction directions and the birefringence values for different light paths through the crystal. The operation of the Bi Ti O crystal in US. Pat. No. 3,374,473 was based primarily upon the difference in extinction directions, as would be shown in FIGS. 4a, 4b, and 4c in a view down the b axis (or a reduced difference in extinction angles in other positions) to detect optically information stored in the ferroelectric domains. The present invention makes use of the birefringence difference obtained by tilting the crystal around the b axis and using incident polarization at approximately 4 5 degrees to the ac plane The difference in birefringence for this orientation may best be under stood from the curves and crystal illustration in FIG. 5. The birefringence curves of FIG. 5 are calculated from the orientation of the optical indicatrix and its principle values, X, Y, and Z shown in FIGS. 4a, 4b, and 4c. The birefringence for the light path in the a-c plane at an angle 6 from c is the difference between Z and the magnitude of the radius of the XY ellipse perpendicular to the internal light path. The birefringence dependence on 0 (tilt angle) can be calculated from X, Y, Z and 0 by conventional algebraic techniques. Table I below gives the experimental data needed to determine X, Y, Z and qb for three wavelengths. For a wavelength different from that given in the table the values of X, Y, Z and can be experimentally determined by conventional optical techniques or extrapolated from the table values.

TABLE I EXPERIMENTALLY DETERMINED PROPERTIES OF THE OPTICAL INDICATRIX X 632.8 nm 589 nm 546.1 nm

4: 29 27 24 A wavelength d angle Y makes with c axis An, birefringence for light path down c An, birefringence for light path down b Since the birefringence is simply the difference in index for the particular section of indicatrix normal to the light path, it can be seen from FIGS. 4 and 5 that a light path exactly down the c axis gives identical birefringence for opposite domains. However, as the crystal is rotated around the b axis to give a light path directed at an angle 6 from c, the birefringence of one domain increases while that of the other decreases, as shown in FIG. 5. These changes are due entirely to the ellipses in the a-c plane since the index Z along the b axis is the same for both domains and is unchanged by the rotation around the b axis. The total retardation R for the two domains is R, An r/cosB 1.

R A72 I/COSQ 2.

where An, and An are the birefringences in the respective domains and are functions of tilt angle expressed in terms of external or internal tilt angle. 0 is the internal tilt angle the angle between the c axis and the light path in the crystal, 0 is the external tilt angle the angle between the c axis and the incident light path as measured external to the crystal (6, is approximately related to 6 by Snells law), I is the crystal thickness (measured along the c axis), and t/cosfl is the optical path length in the crystal.

When the crystal is operated between crossed polarizers and at 45 to the extinction position, then a dark (OFF) state is achieved by choosing t and 0 such that the total retardation for one of the domains is NA for a thickness extinction." Where N is an integer, and A is the wavelength of the light The other domain, in general, will be out of thickness extinction, and for optimum contrast its retardation must be R (N: Any odd multiple of A/ 2 would suffice; however, practical limitations on t and 6 require a difference in retardation of M2. Thus, our design for crossed polarizers is based upon the following equations:

If desired, the design could be based equally well upon parallel polarizers.

The. effect of the crystal tilt 0 and the crystal thickness 1 can be visualized best by graphical consideration of these design criteria. FIG. 6 shows a plot of equations (3) and (4) above for N 6 to 22, together with a plot of (5) for A 632.8nm. This data shows the calculatedthicknesses and tilt angles giving thickness extinction\for the two opposite domains in Bi Ti O (s l d Curves) for. de .6 Q h 221T brok n curve shows the angle and thickness which give an optimum retardation difierence in opposite domains. Any point on the solid curves is a possible operation point; points at the intersection of a solid curve and the broken curve are points of optimum contrast and efficiency. Calculations were made using index values and indicatrix orientation for A 6328 A, as follows; X 2.5984, Y 2.6094, Z 2.7000, and 29. The condition of 5) is not critical, and, in fact, a retardation difference of M4 gives only a small loss (about 3dB) in transmitted light in the ON state. More critical is the condition of (3) or (4) for thickness extinction of one of the domains. Since the changes in birefringence with tilt angle are quite small values superimposed on a large birefringence along 0, the primary variable that can be adjusted to satisfy (3) or (4) is the crystal thickness I. As the crystal is tilted, the birefringence increases in one domain and decreases in the other, as shown in FIG. 5. Also, the tilt results in an increased length in the light path, l/cosfh through the crystal. Thus, it can be seen from the curves for R Nlt that for An these effects tend to cancel, making the thickness-extinction condition quite insensitive to tilt angle. For Am, the tilt has a greater effect since the increased path length and increased An are additive effects.

FIG. 7 is a segment of FIG. 6 with the thickness scale expanded, which shows in more detail the various dereduced contrast and eff ciency.

sign conditions necessary for optimum reading by the differential-retardation method. The NA curves (solid lines minimum transmission) represent the thickness extinction condition or OFF state for each domain for the appropriate order N. The (Ni /9A curves (broken lines maximum transmission) represent the maximum transmission condition or ON state for each domain. Thus, if the domain with a birefringence of An is chosen for the OFF state, then the extinction condition" will be satisfied for any angle and thickness combination on the R NA curve. The intersection of any NA curve and any (Ni /2 curve represents an operating point which gives optimum contrast between oppositely switched domains. These optimum operating points are also represented by the intersection of the NA curves and the curve of equation (5). These optimum operating points are separated by about 3 pm of crystal thickness, which indicates that a uniformity of thickness in the order of ordinary optical tolerances (about A/8) is sufficient to provide uniform contrast over the crystal area. The choice of crystal thickness and tilt angle is dictated to some extent by the resolution, contrast, and efficiency required of the device. For an optimum design in terms of contrast and efficiency, (3) or (4) and (5) must be satisfied. The tilt angle 0 must be limited to about in order to keep the storage plane in reasonable focus and avoid loss of resolution. With this limitation crystal thicknesses of about 90 to 100 um result for the optimum case. Thinner crystals in the range of 30 to ,u.rn (a lower limit for convenient handling) give improved resolution as shown in the following Table II. Thus, if resolution is of primary interest, the requirement of (5) may be relaxed and the device operated with improved resolution but larizer is frequently termed an analyzer. The polarization directions of the polarizers are at 90 as shown in FIG. 2. The polarization directions of the polarizers may be interchanged without effecting the operation. The and 90 degree angle relationships are optimum but excessively critical with only slight degradations occurring for nominal deviations in the accuracy of positioning. The path of the propagating light 25 from the source 14 is first through polarizer 12 where it becomes polarized to the direction 26 (FIG. 2). Without the crystal in place it is to be noted that with the plane of polarization 27 of the second polarizer 13 at 90 degrees to that of the first polarizer no light will pass through the system to the screen 21. The crystal 1 1 is positioned in the light path between the polarizers at 45 degrees from extinction. That is with its a, b, and c axes as shown in FIG. 2. The crystal is then tilted about its b axis through angle 6 until one domain is in extinction, that is, OFF or minimum light in the display. The angle 6 is, as set forth, determined by the retardation of the light through the crystal. The monochromatic light source 14 may be a filtered white light source, a laser or any other conventional monochromatic light source. In a typical embodiment the monochromatic light source was a Ne-I-Ie laser having emission at 632.8 nm. As previously mentioned in the table, when using a laser having polarized light output the first polarizer 12 is not needed. (The polarization direction of the laser emission is properly positioned with respect to the crystal and the second polarizer, i.e., as represented by line 26, FIG. 2.) Other frequencies of monochrorriatic light may be used with the invention, and while the curves shown in FIGS. 6 and 7 will not be quite the same for. other wavelengths since they were calculated for A 6328A the criteria as previously set forth may be appliedito embodiments using monochromatic light RETARDATION READ METHOD Crystal Effithickness Difference in ciency Contrast Resolution \y W m) 7 retardation 6 0 (percent) ratio (lines/mm) 93.3 -A/2(O.45) 26 9.3 12 100:1 -60 37.4 A/6 24 8.7 5 40:l 80-90 measurements.

A typical design of an embodiment may be pursued as follows. Choose R 15A and 0 10.5", then l= 92.9 am is required. This results in \R R l A/2, which gives optimum transmission in the ON state. This is indicated at 71, in FIGS. 6 and 7.

If the exact thickness desired is not obtained, the final adjustment for best thickness extinction can be made by varying the tilt angle 0. A correction for an error in thickness of 0.25 pm can be made by changing the angle 0, by about 1. Of course the best procedure for verifying the thickness (and flatness) is an optical check in monochromatic light, together with a dialgauge measurement to determine the order (N).

In a typical embodiment of a differential retardation information storage and read device of the invention as shown in FIG. 1 the BLTi O plate '11 is positioned between the crossed polarizers 12 and 13. The second poof different wavelengths. For other wavelengths the principal index values of the optical indicatrix can readily be obtained in the conventional manner from measured birefringence values. Curves similar to those shown can be calculated for any other particular wavelength. For example, when using monochromatic light of A 546 nm, and electing to keep approximately the same tilt angle of l0.5 (internal) and a retardation difference of A/2 the corresponding crystal thickness is approximately 78 pm instead of the 92.9 um previously shown to be preferred for the 632.8 um wavelength. The apparatus and operation of the system for write-in and readout is of course the same for all wavelengths, with merely the crystal thickness and the tilt angle being subject to change.

Writing information into the Bi.,Ti O, crystal may be done by any conventional means such as (1 x-y matrix array (2) electron beam addressing, or (3) optical addressing through a photoconductor on the crystal.

These types are well known and fully described in the references previously given. For completeness of the disclosure herein the photoconductor write-in system will be briefly described. In the schematic diagram of an embodiment as shown in FIG. 1 the Bi Ti O crystal 11, has a typica l organ ic photoconduetor 139f polyvinyl carbazole (PVK) doped with 2,4,7-trinitro-9- fluorenone (TNF) spun onto one side of the crystal. (Either side may be used.) Semitransparent gold films 16 and 17 are conventionally deposited on both sides of the photoconductor-ferroelectric sandwich. Contacts 18 and 19 are made in the conventional manner to the respective el ectrodes 1 6 and l], and through them from a conventionaljectrical source the charge that switches the spontaneous polarization of the crystal elements flows. The writing beam 22 and its source and control 23 cooperates with the polarity on the electrodes for placing in the crystal the information to be stored. Information may be written into a crystal with the crystal outside the reading apparatus, and then read out at a later date, or the information may be written into the crystal with the crystal in the reading apparatus. Either contact printing" or object imaging or light beam scanning may be used for writing information into the crystal. Simultaneous read-gut and write-in may be accomplished by,using a read-out light intensity below that required to switch the crys t al or by using a photoconductor; having a low sensitiyity to the read-out fretiuency and using a write-in light of the frequency of maximum photoconductor sensitivity. Generally writein is done separately from read-out, that is, after writein the switching voltage is removed from the electrodes, so that no possible switching will occur during read-out. Then a high intensity read-out light may be used. While a spot beam may be used for read-out, generally a flood beam is used in the complete area of the crystal simultaneously read out. It has been found that some photoconductors perform the switching of the crystal more effectively for one polarity of applied voltage across the photoconductor and crystal than for the opposite polarity. For example, the TNF doped PVK photoconductor has been found to generally have better control of changing the state of individual crystal elements when the electrode next to the PVK layer has a negative potential. (The electrode contacting the crystal on the opposite side of the sandwich is at that time positive.) Thus, it is generally desirable to erase the crystal, that is change it uniformly to one state of light transmission with electrode 16 positive, electrode 17 negative and a high intensity flood beam 22 covering the whole crystal. The crystal will then be in the nontransmitting or OFF" state if the a axis component of P is directed along the positive a axis direction as shown in FIG. 1. It will be the ON state if the a axis component of P, is directed along the negative a axis direction. In either event the state can be transposed by rotating the crystal in a counter-clockwise manner about the b axis by an amount I FIG. 1 shows the crystal tilted or rotated in the clockwise direction about the b axis by an amount [0 Write-in is then made with electrode 16 negative and electrode 17 positive. A suitable source illuminating the crystal for contact printing type write-in has been found to be a ZOO-watt mercury lamp. Flux densities of the write-in (or erase) illumination of the order of SOmW/CM are suitable.

300 to 400 volt switching potentials are also suitable for switching the elements of the crystal for crystal thickness as previously described. (Switching fields of approximately SOKV/cm are generally suitable.) Additional details of photoconductor write-in may be found in the previously mentioned referenced publication appearing in the September 1971 IEEE Transactions on Electron Devices. The x-v matrix arrav type of writingin information is well known and exemplified by the previously referenced US. Pat. No. 3,374,473. The electron-beam writing of information into the crystal is set forth in the publication entitled Electron-Beam Writing of Ferroelectric Domains in Bi Ti O Single Crystals by S. E. Cummins and B. IL Hill appearing in Proceedings Letters pages 938-939 of Proceedings of the IEEE, Vol. 58, No. 6 for June 1970.

The reading of the stored information with monochromatic light may be done with a microscope (visually or photographically), or by projection using a conventional projection lens system 20 and viewing screen 21, with the stored information displayed on the screen.

The compensation embodiment shown schematically in FIG. 3 is similar to the differential-retardation embodiment shown in FIG. 1 except for the addition of a compensator crystal 31 which permits the use of white light (as well as monochromatic light) for read out. The writing of information into the storage crystal and its operation in storing the information is the same as described in the differential-retardation embodiments. The compensator crystal 31 is also a crystal of Ben o like the storage crystal 32. It is positioned in the light path 34 adjacent to the storage crystal 32. Either side of the storage crystal is satisfactory. Both the storage crystal and the compensator crystal must be between the two polarizers. It is to be understood that re fractive and reflective devices, such as lenses, may be inserted anywhere in the optical path, such as between the storage crystal and compensator crystal. Both the Bi Ti O compensator crystal 31 and the Bi Ti O storage crystal-32, as before, are preferably poled for uniform a-axis spontaneous polarization. The optical indicatrix orientations for two domains of opposite c spontaneous polarization are as previously shown in FIGS. 4a, 4b, and 4c. The crystal orientation of primary interest in this embodiment is the same as that for the differential-retardation read embodiment the crystal is out of extinction between crossed polarizers and the path of the incoming light is in the a-c plane at an angle 05x1, from the c axis. Both the differentialretardation embodiment and the present compensation reading embodiment are based upon a difference in the birefringence An, and An which results from the opposite tilt of the indicatrix position in the two domains. In order to take into account the increased path length due to the tilting of the crystal, an effective birefringence, An An/cosfi is used throughout the following description. The total retardation, R, for a 0- plate of thickness d is simply R dAn. The angles 6, and 0 are, as before, the light path angles, as measured from the crystal c axis, internal and external to the crystal, respectively. The calculated variation of effective birefringence as a function of tilt angle 0 for the opposite domains is shown in FIG. 8 as An. and An the variation of birefringence for light in the b-c plane (tilt around the a-axis) is shown as An In the previous differential-retardation read embodiment the off state is obtainedfor one of the domains by making the total retardation equal to an integral number of wavelengths for the monochromatic light used. This results in a thickness extinction for that domain and, with proper design, an out-of-extinction case for the opposite on domain. This type of operation requires highly monochromatic light and precise polishing of the crystal to a predetermined thickness for extinction at the desired operating angle. In this retardation compensation embodiment a second compensating crystal is used which is rotated 90 with respect to the operating crystal. When two crystals of approximately equal thickness are used, their retardation can be made equal but opposite for a compensated "off state for one of the domains. The use of the same material, i.e., bismuth titanate, in the compensator and in the operating crystal results in an excellent black compensation in white-light illumination since dispersion effects cancel and good compensation exists throughout the visible range of wavelengths. The change in total retardation when the operating crystal is switched is adjusted by means of the tilt angle and crystal thickness to give a first-order white for the on state. Generally no electroding or switching of the compensator crystal is used or necessary; however, for some applications (e.g., to electrically produce a negative of a stored pattern), switching of the compensator crystal may be useful. It is satisfactory to use unpoled material in the compensator if the compensator is operated with zero tilt or is tilted around the a axis only. Poled compensator crystals may be tilted around either the a or b axes to achieve proper compensation, or polished to the correct thickness to be used with zero tilt.

Prime considerations in designing the crystal pair for compensated operation are 1) equal and opposite retardations must exist to produce the off state, and 2) the change in retardation with switching should produce the desired first-order white for the on state. A large number of possible thicknesses and tilt-angle combinations will satisfy these requirements; however, the generally preferred choice is the simple case of approximately equal crystal thicknesses and tilts for compensation of like" domains in the compensator and operating crystal for the off state. The operating angle of an experimentally evaluated crystal pair is illustrated in FIG. 8 by the vertical broken line 81 which represents the change in birefringence with switching for an internal tilt angle of 62. This change in An is approximately 0.002; hence, a 100 pm thick crystal will produce a change of 200 nm which has been found to produce a good first-order white. Either An, or Anf can be obtained from the single-domain compensator by simply choosing the direction of tilt. The off state can be obtained by compensating An, of the storage crystal with Anf of the compensator, or similarly Anf of the storage crystal can be compensated by An of the compensator. For parallel light the two cases are equivalent; however, for practical condenser light sources, the incoming cone oflight rays makes the variation of birefringence with angle important since the operation is over a range of angles. From experimental operation, it has been found that the use of An, in both the operating crystal and the compensator gives the best off state. This is as expected since Anf varies less with angle; thus, better compensation exists over the range of angles of the cone of incoming light. Also, the use of a cone of light introduces angles of tilt around the 0 axis, bringing into play the Ari variation. It can be seen from FIG. 8 that An and An are closely matched, which also favors the use of An in the storage-crystal and compensator for the off state.

An estimate of the tolerances that should preferably be maintained in preparing crystals for the compensated read system can be made from observations or birefringence colors ina crystal compensated with a small-angle wedge (or from a chart of birefringence colors). A reasonably dark extinction band exists for a net retardation of: 20 nm. For An 0.10, the i 20 nm is equivalent to a thickness variation of i: 0.2 urn. Thus, it is generally preferable that the angle of tilt of the compensator crystal be adjustable and that the compensator crystal is polished to match the storage crystal, then it is not necessary to reduce the individual crystal tolerance to allow for worst-case variations. It has been found that quite good compensation can be achieved with individual crystal-thickness variations i 0.3 pm.

In a typical operating embodiment of the apparatus as shown schematically in FIG. 3 two Bi Ti O crystals were ground to approximately the same uniform thickness of approximately pm. The storage crystal 32 was positioned at a tilt of approximately 17 about its baxis. It is generally desirable, but not necessary, to be able to vary the angle of tilt slightly to obtain the best compensation with a particular crystal. The compensator crystal 31 has its b axis perpendicular to the b axis 0 the storage crystal. l ik-ewise it is generally desirable to be able to rotate or tilt the compensator crystal slightly about its b axis until maximum contrast is achieved; These minor adjustments are generally easier to perforrrijthan to measure the parameters of the crystal to the accuracy required to determine the exact angles. previously mentioned, with a poled compensator cryst which is generally preferred, it may be tilted around ei her the a or b axis to achieve maximum contrast. Note thatthe b axis of the two crystal in both instances remain perpendicular. (When the compensator crystal is not poled the tilting is limited to only around the a axis.) The light source 33 in atypical embodiment was a conventional 100 watt tungsten lamp with a heat filter and a relay-type condenser system.

FIG. 9 shows the measured light output for on and off domains and the contrast ratio for two cases of compensation. In the first case represented by curves 91, 92, and 93 (-on, off, and contrast ratio, respectively) the preferred An as previously stated and as described, was used. The crystals were then oriented slightly so as to operate in the An, orientation and the respective measurements then produced curves 94, 95, and 96. The preferred An orientation can be seen to give considerably higher contrast ratios mainly because of the more effective compensation in the off state. This is due to the shape of the buefringence-vs-O relationship as represented by the curves of FIG. 8. The relative flatness and nearly equal magnitudes of Anf and An result in more effective compensation throughout the range of angles of the incoming cone of light which, in the particular embodiment from which the data of FIG. 9 was obtained, had a total angle of about 30.

The a, b, and c axes referred to herein are the crystallographic axes of the crystal. The physical crystal may be cut along these axes as shown in the drawing, however, this is not a requirement; The crystals as grown are normally in the form of flat plates with the c axis perpendicular to the major surfaces. It is the thickness of the crystal, i.e., the dimension along the c axis, that is the controlling dimension as taught herein. Round or irregular shaped crystals in the a-b plane may just as satisfactorily be used.

While reference has been made herein to imply that the angle between the a and c axes is a right angle, it is to be understood that in the actual monoclinic structure of Bi Ti O crystals these axes do not intersect at exactly 90 degrees, however the deviation is so small that for practical purposes as used herein it is negligible.

We claim:

1. Apparatus for reading an image stored as ferroelectric domain patterns in a Bi Ti O single crystal comprising:

a. means for providing a path of polarized monochromatic propagating light;

b. a polarizer having a polarization direction positioned in the said path of light, the said polarizer having its polarization direction rotated about the path of light substantially 90 degrees from the polarization direction of said polarized propagating light;

c. a single crystal of Bi Ti O having the said image stored as ferroelectric domain patterns positioned in the propagating light path between the said polarizer and the said means for providing a path of polarized propagating light, at substantially 45 degrees from extinction and tilted about its b axis to provide extinction for one of the said domains; and

d. means for displaying the said light passing through the said second polarizer displaying the said image.

2. The apparatus as claimed in claim 1 wherein the a-c plane of the said crystal is parallel to the said path of propagating light.

3. The apparatus as claimed in claim 1 wherein the said single crystal of Bi Ti- O has a total retardation for one of the domains equal to an integral number of wavelengths of the said monochromatic light.

4. The apparatus as claimed in claim 1 wherein the said display means comprises a projection lens and viewing screen.

5. The apparatus as claimed in claim 1 wherein the said means for providing a path of polarized monochromatic propagating light is a laser providing a path of polarized monochromatic propagating light.

6. The apparatus as claimed in claim 1 wherein the said means for providing a path of polarized propagating light is a light source providing a path of propagating monochromatic light and a polarizer having a polar- 12 ization direction positioned in the said path of propagating light.

7.. Apparatus for reading an image stored as ferroelectric domain patterns in a Bi Ti O single crystal comprising:

a. a light source providing a path of propagating light;

b. a first polarizer having a direction of polarization positioned in the said light path adjacent the light source polarizing the light passing therethrough;

c. a second polarizer positioned in the said light path having its direction of polarization rotated about the path of light, degrees from the plane of polarization of the said first polarizer;

d. a first single crystal of Bi Ti O poled for substantially uniform a axis spontaneous polarization, and having said image stored therein in domains of opposite c spontaneous polarization, positioned between the said first and second polarizers, in the propagating light path with the a-c plane of the said crystal parallel to the said path of propagation, rotated in the said path of light 45 degrees from extinction and tiltedabout its b axis to a retardation difference in the said domains of opposite spontaneous. polarization;

e. a second single crystal of Bi Ti O positioned between the-said first and second polarizers in the said path oflight with the b axis of the second crystal perpendicular to the b axis of the first crystal with the b-c plane of the second crystal substantially parallel to the said path of the propagating light, the said secoiidcrystal having a retardation to the light passing therethrough such that the total retardation to the lightpassing through the second crystal and one domain in the first crystal is substantially zero; and

f. means cooperating with the said light passing through the said second polarizer for displaying the saidimage stored in the first crystal.

8. The apparatus as claimed in claim 7 wherein the c axis of the second crystal is parallel to the light path and the thicknessalong the c axis of the said second crystal is such that the said total retardation for one domain of the first crystal is zero.

9. The apparatus as claimed in claim 7 wherein the said second crystal is tilted about the a axis such that the said total retardation for one domain is zero.

10. The apparatus as claimed in claim 7 wherein the said second crystal is poled for substantially uniform spontaneous polarization.

11. The apparatus as claimed in claim 10 wherein the said second crystal is tilted about the b axis with the a-c plane of the second crystal substantially parallel to the path of propagation such that the said total retardation for one domain is zero. 

2. The apparatus as claimed in claim 1 wherein the a-c plane of the said crystal is parallel to the said path of propagating light.
 3. The apparatus as claimed in claim 1 wherein the said single crystal of Bi4Ti3O12 has a total retardation for one of the domains equal to an integral number of wavelengths of the said monochromatic light.
 4. The apparatus as claimed in claim 1 wherein the said display means comprises a projection lens and viewing screen.
 5. The apparatus as claimed in claim 1 wherein the said means for providing a path of polarized monochromatic propagating light is a laser providing a path of polarized monochromatic propagating light.
 6. The apparatus as claimed in claim 1 wherein the said means for providing a path of polarized propagating light is a light source providing a path of propagating monochromatic light and a polarizer having a polarization direction positioned in the said path of propagating light.
 7. Apparatus for reading an image stored as ferroelectric domain patterns in a Bi4Ti3O12 single crystal comprising: a. a light source providing a path of propagating light; b. a first polarizer having a direction of polarization positioned in the said light path adjacent the light source polarizing the light passing therethrough; c. a second polarizer positioned in the said light path having its direction of polarization rotated about the path of light, 90 degrees from the plane of polarization of the said first polarizer; d. a first single crystal of Bi4Ti3O12 poled for substantially uniform a axis spontaneous polarization, and having said image stored therein in domains of opposite c spontaneous polarization, positioned between the said first and second polarizers, in the propagating light path with the a-c plane of the said crystal parallel to the said path of propagation, rotated in the said path of light 45 degrees from extinction and tilted about its b axis to a retardation difference in the said domains of opposite spontaneous polarization; e. a second single crystal of Bi4Ti3O12 positioned between the said first and second polarizers in the said path of light with the b axis of the second crystal perpendicular to the b axis of the first crystal with the b-c plane of the second crystal substantially parallel to the said path of the propagating light, the said second crystal having a retardation to the light passing therethrough such that the total retardation to the light passing through the second crystal and one domain in the first crystal is substantially zero; and f. means cooperating with the said light passing through the said second polarizer for displaying the said image stored in the first crystal.
 8. The apparatus as claimed in claim 7 wherein the c axis of the second crystal is parallel to the light path and the thickness along the c axis of the said second crystal is such that the said total retardation for one domain of the first crystal is zero.
 9. The apparatus as claimed in claim 7 wherein the said second crystal is tilted about the a axis such that the said total retardation for one domain is zero.
 10. The apparatus as claimed in claim 7 wherein the said second crystal is poled for substantially uniform spontaneous polarization.
 11. The apparatus as claimed in claim 10 wherein the said second crystal is tilted about the b axis with the a-c plane of the second Crystal substantially parallel to the path of propagation such that the said total retardation for one domain is zero. 